1. Field of the Invention
The present invention relates to an optical member for photolithography, adapted for use in an optical system such as lenses, mirrors, and the like in a specific wavelength region of 400 nm or less and, preferably, 300 nm or less in an ultraviolet (UV)-lithography technique.
The present invention also relates to a method for evaluating an optical member for photolithography.
2. Related Background Art
In recent years, VLSIs have shown remarkable progress in the level of integration and in their functions. In the field of logic VLSIs, there is being developed the system-on-chip concept, incorporating a large system on a chip. With such tendencies, there is being required finer patterning and higher integration on the substrate such as silicon wafer. In a photolithography technique for exposing and transferring fine patterns of an integrated circuit onto a wafer such as silicon, there is employed an exposure apparatus called a stepper.
As an example, in the field of DRAM, along with advancement from LSI to VLSI, or with the increase in capacity 1K.fwdarw.256K.fwdarw.1M.fwdarw.4M.fwdarw.16M, there are required steppers capable of reproducing progressively decreasing the line width of patterns 10 .mu.m.fwdarw.2 .mu.m.fwdarw.1 .mu.m.fwdarw.0.8 .mu.m.fwdarw.0.5 .mu.m.
For this reason, a projection lens of the stepper is required to have a high resolution and a large focal depth. The resolution and the focal depth are determined by the wavelength of light used for exposure and the numerical aperture (N.A.) of a lens.
As the pattern becomes finer, the angle of diffracted light increases, and it becomes difficult to receive the diffracted light unless the lens has a large N.A. On the other hand, as the exposure wavelength .lambda. is shorter, the angle of diffracted light becomes smaller if the pattern remains the same size, so that, the N.A. can be small.
The resolution and the focal depth are represented as follows: EQU Resolution=k1.multidot..lambda./N.A. EQU Focal Depth=k2.multidot..lambda./N.A..sup.2
(where k1 and k2 are coefficients of proportion.)
An improvement in the resolution can be achieved by an increase in N.A. or by a reduction in .lambda.. As can be seen from the above equations, the reduction in .lambda. is more advantageous in consideration of the focal depth. From this viewpoint, the wavelength of a light source is being shortened from the g-line (436 nm) to the i-line (365 nm), and further to a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm).
An optical system incorporated in a stepper is composed of a combination of a large number of optical members such as lenses. Even if the amount of light loss in the transmittance per a single lens is small, it is accumulated by the number of lenses, and the accumulated light loss leads to a significant decrease in illumination intensity. For this reason, each optical member is required to have a high transmittance.
Thus, in a short wavelength region of 400 nm or less, optical glass manufactured by a special method which takes light loss due to short wavelength and combination of optical members into account is employed. Furthermore, it is proposed to use synthesized silica glass or monocrystalline such as CaF.sub.2 (fluorite) in a wavelength region of 300 nm or less.
On the other hand, in order to realize a smaller line width and to obtain fine and sharp exposure patterns, or fine and sharp transfer patterns, it is indispensable to obtain as a projection lens an optical member which has a highly homogeneity of refractive index (a small fluctuation of refractive index in a measurement range). However, due to recent enlargement of exposure area along with enlargement of semiconductor wafer, the aperture size and thickness of such a material are increasing, and it is becoming harder to obtain the above-mentioned quality of the material. Thus, in order to improve the homogeneity of the refractive index of an optical member with a large aperture size and thickness, various attempts have been made.
The homogeneity of the refractive index has been conventionally represented by the difference between the maximum and minimum values of the refractive index with the measurement range (to be referred to as .DELTA.n hereinafter), and an optical member is considered to have higher homogeneity as this value becomes smaller. For this reason, existing "high-homogeneity" optical members are manufactured to minimize .DELTA.n.
However, a fine, sharp exposure-transfer pattern cannot often be obtained, even though an optical member with sufficiently small .DELTA.n, e.g., an optical member on the 10.sup.-6 order or less, which is generally considered as a high-homogeneity optical member, is used.
The present inventors have proposed a photolithography silica glass member (U.S. patent application Ser. No. 193,474) which is used in a specific wavelength range of 400 nm or less, and in which the RMS (root mean square) value of wavefront aberration is 0.015.lambda. or less after the removal of tilt power, the slant element of the refractive index is .+-.5.times.10.sup.-6 or less, or the refractive index distribution is rotationally symmetrical with an axis, and the axis of rotationally symmetric is coinciding the central axis of the silica glass member.